Just when you thought you had mastered Zo, the characteristic impedance of a PCB trace, along comes a data sheet
that tells you to design for a specific differential impedance.
And to make things tougher, it says things like: “… since the
coupling of two traces can lower the effective impedance, use
50 Ohm design rules to achieve a differential impedance of
approximately 80 Ohms!” Is that confusing or what!!
This article shows you what differential impedance is.
But more than that, it discusses why it is, and shows you how
to make the correct calculations.
Single Trace:
Figure 1(a) illustrates a typical, individual trace. It has a
characteristic impedance, Zo, and carries a current, i. The
voltage along it, at any point, is (from Ohm’s law) V = Zo*i.
General case, trace pair:
Figure 1(b) illustrates a pair of traces. Trace 1 has a
characteristic impedance Z11, which corresponds to Zo,
above, and current i1. Trace 2 is similarly defined. As we
bring Trace 2 closer to Trace 1, current from Trace 2 begins
to couple into Trace 1 with a proportionality constant, k.
Similarly, Trace 1’s current, i1, begins to couple into Trace 2
with the same proportionality constant. The voltage on each
trace, at any point, again from Ohm’s law, is:
V1 = Z11 * i1 + Z11 * k * i2 Eqs. 1
V2 = Z22 * i2 + Z22 * k * i1
Now let’s define Z12 = k*Z11 and Z21 = k*Z22. Then,
Eqs. 1 can be written as:
V1 = Z11 * i1 + Z12 * i2 Eqs. 2
V2 = Z21 * i1 + Z22 * i2
This is the familiar pair of simultaneous equations we
often see in texts. The equations can be generalized into an
arbitrary number of traces, and they can be expressed in a
matrix form that is familiar to many of you.
Special case, differential pair:
Figure 1(c) illustrates a differential pair of traces. Repeating Equations 1:
V1 = Z11 * i1 + Z11 * k * i2 Eqs. 1
V2 = Z22 * i2 + Z22 * k * i1
Now, note that in a carefully designed and balanced situation,
Z11 = Z22 = Zo, and
i2 = -i1
This leads (with a little manipulation) to:
V1 = Zo * i1 * (1-k)
博客介绍了信息技术中常见接口的阻抗值，其中USB阻抗为90欧，LVDS和HDMI阻抗均为100欧。
扫一扫
690
被折叠的 条评论
为什么被折叠?
到【灌水乐园】发言
